FACTORS INFLUENCING THE REBOUND HAMMER RESULTS

Factors Influencing the Results
Type of cement
Concrete made of high alumina cement can give strengths up to 100% higher, whereas super sulphated cement concrete can give 50% lower strength compared to a calibration obtained on Portland cement cubes. It is necessary to recalibrate the hammer for different types of cement.

Type of aggregate

Gravel and most crushed rocks give similar correlations, but lightweight aggregates and aggregates with unusual properties require special calibration.

Surface and internal moisture condition of the concrete.

This method of testing is applied only on close textured concrete. Open texture concrete typical of masonry blocks, `honeycombed’ concrete, or no fines concrete cannot be tested using this method.

Trowelled and floated surfaces as in floors, are harder than moulded surfaces and in most cases will tend to overestimate the strength.

A wet surface will give rise to under-estimated of the strength of concrete calibrated under dry conditions. This influence can be considerable and in structural concrete it is about 10% lower on wet surfaces than on an equivalent dry surface.

Age of concrete
In a very old and dry concrete the surface will be harder than the interior, giving rebound values somewhat higher than normal. New concrete with moist surface generally has a relatively softer surface, resulting in lower than normal rebound.